A packet data gateway (PDG) functionality may be performed using a tunnel termination gateway (TTG) and a gateway GPRS support node (GGSN) as specified in 3GPP TS 23.234 appendix F. In this case, the TTG assigns a network service access point identifier (NSAPI), which is used to identify a General Packet Radio Service (GPRS) tunneling protocol (GTP) tunnel between the TTG and GGSN. This is different from previous processes where the mobile device assigned the NSAPI. In the 3GPP TS 23.234 appendix F architecture, a single TTG is associated with a single GGSN.
The problem with using a one-to-one mapping between a TTG and GGSN is that scalability is not provided. For example, when multiple GGSNs are being used, the load at one GGSN may be more than another GGSN, which may cause an overloaded GGSN to end a session even when another GGSN has available resources. However, because of the one-to-one mapping, a TTG cannot send messages to the underutilized GGSN. Rather, the overloaded GGSN still must be used.
To increase scalability, load balancing may be used to spread use of GGSN resources to underutilized GGSNs. However, because the TTG assigns the NSAPI, it is possible that the same NSAPI may be sent to the same GGSN by different TTGs. In this case, a tunnel may already be associated with the NSAPI. Then, a second TTG may assign the same NSAPI to a new tunnel for the same GGSN. In this case, the GGSN will tear down the first tunnel and initiate a new tunnel using the new NSAPI. This may end an active session prematurely.